The invention relates to methods and devices for separating, within a fluid, certain particles contained therein, such as, in particular, colloidal particles (pigments, silica or gold grains, latex, etc.) and biological particles (viruses, cells, bacteria, proteins, DNA fragments, etc.) contained in a liquid, or alternatively grains or balls with small diameter (diameters lying, for example, between 0.5 and 5 mm), made of metal, plastic, mineral (silica, etc.), contained in a gas, such as ambient air, or alternatively the components of a gas mixture (oxygen, nitrogen, etc.), or even isotopes contained in a gas, such as the isotopes of uranium contained in the hexafluoride of this metal, or the isotopes of hydrogen (deuterium, tritium), of lithium, of boron, of gadolinium, etc.
The invention more particularly relates to the technique according to which a separation or entrainment field is applied to the fluid in question, contained in a suitable chamber, which field is capable of acting specifically on the particles to be separated so as to displace them from a first zone of the chamber to another zone in which the concentration of these particles increases because of these displacements.
Solutions hitherto proposed for this technique are essentially of three types.
In solutions of the first type, the entrainment field is applied onto the fluid along the actual entrainment direction, between two ends, far apart from each other, of the chamber, and respectively corresponding to its "inlet", or zone intended to be depleted in the particles to be separated, and its "outlet", or zone intended to be enriched in the said particles: such a solution requires a relatively high value for the field, and the increase in this field value is rapidly limited by the price of the corresponding equipment.
In solutions of the second type, the entrainment is obtained by creating an electric wave propagating over the entire length of the chamber (see the article by R. Hagedorn, G. Fuhr, T. Muller and J. Gimsa appearing in pages 49-54 of Volume 13, (1992) of the journal "Electrophoresis").
For this, the said chamber is at least partially given the shape of a corridor partly defined by two faces which are close together and substantially parallel to each other and parallel to the direction E of entrainment or separation and the polarizing field is applied on the volume of fluid contained in the said corridor, along a direction perpendicular to the direction E.
In the embodiments proposed for implementing this second type of solution, the fields in question are electric fields and they must be sequentially distributed between a large number of separate electrodes which are insulated from each other and arranged side by side along the entire length of the corridor.
The resulting equipment is complex and difficult to control.
In the third type of solution, the fluid to be treated is a liquid which is placed in a corridor which is slightly inclined to the horizontal and vertically defined by two parallel electrode plates which are regularly corrugated in symmetric patterns, the projecting zones of the plates being arranged mutually opposite in pairs so as to create a succession of constricted zones and of widened zones (Patent AU-B-37 633/78).
The liquid flows under gravity between the plates because of their inclination and a uniform electric potential is applied between the two plates, which exerts differentiated electrical stresses on the various particles contained in the constricted zones: these differentiated stresses result in a progressive enrichment of the liquid flowing between the plates in certain of the said particles.